1. Field of the Invention
The present invention relates to magnetic random access memory, and more particularly to the efficient selection and switching of a magnetic element within a magnetic random access memory array.
2. Background Art
The desired characteristics for computer memory are high speed, low power consumption, non-volatility, high data density and low cost. Dynamic Random Access Memory (DRAM) cells are fast and expend little power, but have to be refreshed many times each second and require complex structures which can make them relatively expensive. Flash type EEPROM cells are nonvolatile, have low sensing power, and can be constructed as a single device, but take microseconds to write and milliseconds to erase, which makes them too slow for many applications, especially for use in computer memory. Conventional semiconductor memory cells such as DRAM, ROM, and EEPROM have current flow in the plane of the cell, i.e., xe2x80x9chorizontalxe2x80x9d, and therefore occupy a total surface area that is the sum of the essential memory cell area plus the area for the electrical contact regions, and therefore do not achieve the theoretical minimum cell area
Magnet Random Access Memory is a promising candidate for computer memory that can meet the above stated objectives while overcoming many of the limitations of the above described devices. The benefits of Magnetic Random Access Memory (MRAM) are discussed in xe2x80x9cThe Science and Technology of Magnetoresistive Tunneling Memoryxe2x80x9d, IEEE Transactions on Nanotechnology, Vol. 1, No. 1, March 2002, by B. N. Engel et al. A Magnetic Random Access Memory is essentially a grid of electrically conductive bit lines and word lines. The bit lines are parallel to one another and are perpendicular to the word lines, which are also parallel with one another. A magnetoresistive cell, disposed at the intersection of each word and bit line, electrically connects a particular word line to a particular bit line. By applying a voltage across the magnetoresistive cell from the particular word line to the particular bit line, the magnetic resistance, and memory state, of the magnetoresistive cell can be determined. Although various types of magnetoresistive cells can potentially be used in a MRAM array, such as for example a Current Perpendicular to Plane Giant Magnetoresistance Element (CPP GMR), most development efforts have focused on the use of Tunnel Valves also known as Magnetic Tunnel Junction (MTJ) cells, due to their potential for high resistance change, xcex4R/R. Therefore, while the state of the MRAM art will be described with reference to MTJ cells, it should be appreciated that many of the same principles and challenges apply to MRAM arrays incorporating other types of magnetoresistive cells as well.
A MTJ cell, in its most general sense includes first and second ferromagnetic layers separated by a thin insulating layer known as a tunnel barrier layer. One of the ferromagnetic layers has its magnetization pinned in a predetermined direction while the other is free to rotate under the influence of magnetic field. Quantum mechanical tunneling of electrons through the tunnel barrier layer is allowed or inhibited based on the relative alignments of the magnetization of the free and pinned magnetic layers. If the magnetization of the free layer is aligned parallel with that of the pinned layer, then electrons will tend to pass through the tunnel barrier layer; if the magnetizations of the two layers are anti-parallel, then electrons will not as easily pass. Furthermore, the MTJ is designed so that the magnetically free layer will have a magnetic anisotropy, which will cause the magnetization of the free layer to be most stable when in either of these two states, (ie. parallel or anti-parallel to the pinned layer). In this way, once the MTJ cell is placed in a particular magnetic state it will tend to remain there until acted on by a magnetic field, thus providing its non-volatility. Switching of the MTJ cell is generally accomplished by causing a current to flow through both of the word and bit lines.
The free layer of the cell at the intersection of the energized bit and word lines will undergo magnetization reversal, provided that the magnetic fields produced by the word and bit lines are sufficiently high. The Stoner-Wohlfarth coherent rotation model quite accurately describes the switching behavior of a typical free layer in the MRAM cell. According to the model, the switching will occur if the magnetic fields in the hard and easy axis directions lie outside of the so-called astroid curve, as shown in FIG. 4A. The operating currents are chosen such that they produce a magnetic field at the selected cell that satisfies the switching conditions for the cell, but not for any other cell along word or bit lines (i.e. the fields for non-selected bits have to lie within the astroid curve). However, the cells in the array do not have identical magnetic properties. Variations in the bit size, shape, aspect ratio, and crystalline anisotropy lead to a distribution of the switching fields. This leads to a problem regarding bit selectivity, i.e., for a given write current, some of the selected bits will switch, while others might not. Simply increasing the write current would allow switching of these hard-to-switch bits; however, some of the non-selected bits will switch with such large currents. This is illustrated in FIG. 4B. The inner and outer astroids (Amin and Amax, respectively) show the extremes in the switching distributions of the free layer in the MRAM array. If the operating point O is chosen within Amax, then only cells within astroid A can be switched, while others outside astroid A cannot be addressed. To address these cells, the operating point has to be moved to point P outside of Amax. But in such a case, all the cells with distribution described by Amin that lie along the selected word or bit lines, will be unintentionally switched.
Another factor that adversely affects bit selectivity, is the magnetic field produced by the neighboring cells. These fields can change the fields required to switch the selected bit at the chosen operating point or lead to unintentional switching of the non-selected bits. The problem of the bit selectivity is widely recognized in the field, and numerous solutions have been proposed, such as the use of thermally assisted writing as described in U.S. Pat. No. 6,385,082 by D. W. Abraham et al., the use of offset bit currents as described in U.S. Pat. No. 6,424,561 by Shaoping Le et al., and the use of magnetic bias as described in U.S. Pat. No. 6,163,477 by L. T. Tran. However, the proposed solutions generally compromise some other desirable aspects of MRAM, such as power consumption or areal density.
Another problem encountered in MRAM is the magnetic stability of the bits. As the size of the bits become smaller, they approach the superparamagnetic limit. To increase the stability, one solution requires increasing the magnetic anisotropy of the bits, but this also requires higher magnetic fields to switch the bits and in turn, requires larger power consumption.
The present invention allows a particular magnetoresistive cell within a MRAM array to be selected and switched with a minimum of power and also allows a particular cell to be selected for switching while leaving other adjacent cells unaffected. The invention takes advantage of the ferromagnetic resonance frequency of the cell. By applying a DC current to one of the lines connected to the cell, the resonance frequency of the cell is shifted. An AC signal is then applied to the other of the lines connected to the particular cell. This AC signal is generated at a frequency that is generally the same as the shifted resonance frequency of the selected cell and creates a magnetic field at that same frequency. This magnetic field causes the magnetization of the free layer to oscillate at its shifted resonance frequency, rendering it easily switched with a minimum of energy.
The present invention provides significant advantages over the prior art for at least a couple of reasons. First, according to one aspect of the invention, a selected magnetoresistive cell can be switched from one magnetic/resistance state to another with a minimum of input energy. As discussed with reference to the prior art, the MRAM array includes a grid made up of a set of parallel word lines and a set of parallel bit lines, the word lines and bit lines being generally perpendicular to one another. Each specific word line is connected with a specific bit line by a magnetoresistive cell. The magnetic state of the cell can be controlled by causing a current to flow through the word and bit lines associated with the selected cell. The currents generate magnetic fields, which act upon the magnetic moment of the free layer to rotate it from one direction to another. By applying an AC current to a word line at the ferromagnetic resonance frequency, the magnetizations of the cells along the word line are rotated by a larger angle than with a DC current of the same amplitude. Additionally, by applying a DC current to a bit line, the ferromagnetic resonance frequency is shifted for all cells along the bit line, thus allowing for greater selectivity of the cell at the intersection with the word line.
Second, according to another aspect of the invention the resonance frequency of the particular selected cell can be shifted so that, by applying the above described AC signal at this shifted frequency, the selected cell will oscillate at the resonance frequency resulting in large magnetization rotation while other adjacent cells whose resonance frequency has not been shifted will oscillate with a much smaller amplitude. Shifting of the resonance frequency of the selected cell can be accomplished by generating a current having a DC bias through a bit line. By way of example, a predetermined DC current can be applied to the bit line. This generates a magnetic field along the easy axis of the free layer which shifts the resonance frequency of the cells along the bit line as approximately described by the formula             f      FMR        =                  γ                  2          ⁢                      xe2x80x83                    ⁢          π                    ⁢                        4          ⁢                                    π              ⁢              M                        (                                          H                Keff                            ±                              H                ext                                      )                                ,
where xcex3 is the gyromagnetic constant, M is the saturation magnetization of the free layer, HKeff is the effective anisotropy of the free layer, which can include both the crystalline anisotropy and the shape anisotropy, and Hext is the external field from the bit line. The above equation assumes that both the magnetization and the external field lie in the plane of the free layer and along the easy axis. The external field may be either parallel or antiparallel to the magnetization.
Then an AC current having a frequency that is essentially the same as the shifted resonance frequency of the selected cell can be applied to the word line associated with that cell, generating a magnetic field of the same frequency. It will be appreciated that, even though all of the cells on that word line will be exposed to the oscillating magnetic field, and all of the cells on the bit line will experience a constant magnetic field Hext, the cell at the intersection of the word and bit lines will have by far the largest magnetization rotation since it will be the only cell being driven at resonance.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures, wherein like reference numerals refer to like elements.